Possible explanation for the unexpected absence of gross biological damage to membranes of cells insonated in suspension and in surface culture in chambers exposed to standing and progressive wave fields.

Quoted values of cavitation thresholds reported in the literature vary over several orders of magnitude. This paper describes an investigation of the threshold in a chamber with acoustically transparent windows, situated around the last axial maximum of a 0.75 MHz standing wave field. The chamber, the transducer and the reflector are submerged in a tank containing distilled water which also fills the chamber. The formation of bubbles outside the chamber, both above and below it, occurred at lower intensities than those needed to generate bubbles in the chamber. This unexpected finding led to a theoretical study of streaming patterns in progressive and standing wave fields. These showed that there is a region around the last axial maximum in which there is no gradient of energy density. Therefore if this region is enclosed by an acoustically transparent chamber, no significant bulk streaming occurs within it. We speculate on how this lack of streaming raises the cavitation threshold. The cavitation phenomenon is also examined by Doppler ultrasound of frequency 8 MHz. This confirms the occurrence of increased intensity thresholds within the chamber. It also shows that the time between filling the chamber and the start of sonication strongly influences the magnitude of the cavitation threshold. We expect that the effects described here may have consequences for sonication of cells in suspension culture when the samples are held in chambers situated around the last axial maximum of an ultrasound beam from a plane transducer.